Structure of Reverse Micelle and Microemulsion Phases in Near

Chapter 12

Downloaded by UNIV OF LIVERPOOL on September 19, 2016 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch012

Structure of Reverse Micelle and Microemulsion Phases in Near-Critical and Supercritical Fluid as Determined from Dynamic Light-Scattering Studies Richard D. Smith, Jonathan P. Blitz, and John L. Fulton Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, WA 99352 Dynamic light scattering methods were used to study reverse micelle and microemulsion phases formed in liquid and supercritical alkane continuous phases. These reverse micelle or microemulsion (w/o) phases were formed in the alkanes from ethane through decane using the surfactant aerosol-OT (AOT) with variable amounts of water. A high-pressure cylindrical sapphire cell, capable of operation at pressures up to 500 bar and over 80°C, was used for measurements of both single-phase and the upper (alkane continuous) phase of two-phase systems. Measured changes in hydrodynamic diameter of reverse micelle or microemulsion phases can be largely attributed to attractive micelle-micelle interactions. Such interactions become increasingly evident at solution pressures approaching the phase boundary between the one-phase and two-phase systems and are also enhanced at higher surfactant and water concentrations. It is shown that reverse micelles exist in the upper fluid phase of two-phase systems, and have sizes (or W values) which are highly pressure dependent. Several years ago we reported i n i t i a l observations of reverse m i c e l l e s and microemulsions i n s u p e r c r i t i c a l f l u i d solvents (D · These studies suggested the p o s s i b i l i t y of c r e a t i n g a previously unsuspected broad range of organized molecular assemblies i n dense gas s o l v e n t s . Such systems are of i n t e r e s t due t o p o t e n t i a l a p p l i c a t i o n s which e x p l o i t the r e a d i l y v a r i a b l e p r o p e r t i e s of s u p e r c r i t i c a l f l u i d s as well as the unique solvent environments of reverse micelles and microemulsions. These i n i t i a l studies showed that even gram q u a n t i t i e s of proteins, such as Cytochrome-c (Mwt. 12,842 dalton) could be s o l v a t e d i n a l i t e r of s u p e r c r i t i c a l ethane or propane due t o the microemulsion solvent environment, something which i s not a c h i e v a b l e with "conventional" 0097-6156/89/0406-0165$06.00A) © 1989 American Chemical Society

Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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s u p e r c r i t i c a l f l u i d s due to the l i m i t e d p o l a r i t y and p r a c t i c a l temperature constraints. The formation of reverse m i c e l l e s and w a t e r - i n - o i l (w/o) microemulsions i n l i q u i d hydrocarbons using the surfactant sodium b i s ( 2 - e t h y l h e x y l ) s u l f o s u c c i n a t e (AOT) has been widely studied (2xJl) . In nonpolar l i q u i d solvents, these molecular aggregates g e n e r a l l y c o n s i s t of 3to 20-nanometer-diameter, roughly s p h e r i c a l s h e l l s of surfactant molecules surrounding a polar core, which i s t y p i c a l l y an aqueous s o l u t i o n . This combination of h y d r o p h i l i c , hydrophobic, and i n t e r f a c i a l environments i n one solvent has created p o t e n t i a l a p p l i c a t i o n s i n separations (JUL), chromatography (£), and c a t a l y t i c reactions (2) . In conventional l i q u i d microemulsions, the properties of the s u r f a c t a n t i n t e r f a c i a l r e g i o n are of primary importance i n determining the s i z e and shape of s u r f a c t a n t aggregates. The i o n i c i n t e r a c t i o n s between surfactant head groups and i n t e r a c t i o n s with t h e i r counter ions, as w e l l as the hydrogen bonding within the aqueous core, are important factors which lead to aggregation. The e q u i l i b r i u m s t r u c t u r e of the i n t e r f a c i a l region i s a l s o determined by a d e l i c a t e balance of s e v e r a l a d d i t i o n a l f a c t o r s . Small changes i n the surfactant's hydrocarbon t a i l structure (JL) as w e l l as small changes i n the composition of the nonpolar solvent (8,9) cause profound changes i n the aggregate structure or s i z e as evidenced by the large changes i n the amount of water or surfactant (and other substances) which can be s o l u b i l i z e d by the microemulsion. Previous r e s u l t s have i n d i c a t e d that aggregation or c l u s t e r i n g of two or more m i c e l l e s can a l s o occur when the m i c e l l e s are of s u f f i c i e n t s i z e or at high enough concentrations (1Û). Indeed, the phase behavior of c e r t a i n m i c e l l a r solutions i s known to resemble that of a simple molecular f l u i d which has l i q u i d - g a s phase e q u i l i b r i a and a w e l l - d e f i n e d c r i t i c a l point (11,12).

The a b i l i t y of surfactants such as AOT (usually with water) to form m i c e l l e and microemulsion phases i n s u p e r c r i t i c a l f l u i d s (dense gases) (1.13) opens up a range of p o t e n t i a l new applications. A s u p e r c r i t i c a l f l u i d i s a substance above i t s c r i t i c a l temperature and pressure which has p r o p e r t i e s that are highly dependent on pressure due to the proximity to the c r i t i c a l point. In s u p e r c r i t i c a l f l u i d s density, d i e l e c t r i c constant and v i s c o s i t y , as well as other properties, can be continuously varied between the gas and l i q u i d phase l i m i t s by manipulating pressure. F l u i d s , i n which AOT can be used to create micelles and Which are s u p e r c r i t i c a l at moderate temperatures and pressures i n c l u d e ethane (T - 32.2°C, P - 48.8 bar), xenon (T - 16.6°C, P - 58.4 b a r ) , and propane (T - 96.7°C, P - 42.4 b a r ) . Microemulsions formed i n s u p e r c r i t i c a l f l u i d s have been previously characterized by light s c a t t e r i n g (14.15), s p e c t r o s c o p i c methods (UL), conductivity, density, and phase b e h a v i o r (UL) · These s u r f a c t a n t / s u p e r c r i t i c a l f l u i d systems have p o t e n t i a l applications i n enhanced o i l recovery (16) , r e a c t i o n processes (17-19) t chromatography (20.21), and bulk separations processes (22.) where the high d i f f u s i v i t i e s of solutes i n the f l u i d continuous phase may g r e a t l y increase reaction or extraction rates. c

c

c

c

c

c



12.

SMITH E T A L .

Structure ofReverse Micelle and Microemulsion Phases

The p r o p e r t i e s o f a s u p e r c r i t i c a l f l u i d continuous phase provide a u s e f u l t o o l with which t o study surfactant aggregation. Haydon and coworkers (22.) have shown that a low molecular weight l i q u i d alkane, e.g., butane, p e n e t r a t e s and s o l v a t e s t h e hydrocarbon t a i l region o f the s u r f a c t a n t i n t e r f a c e t o a much g r e a t e r extent than h i g h e r molecular weight l i q u i d s , e.g., hexadecane. Smaller molecules, such as xenon or ethane, might then be expected t o possess the c a p a b i l i t y of even g r e a t e r s o l v a t i o n of t h i s hydrocarbon t a i l region (15.) . I t i s also expected that the degree of s o l v a t i o n of the hydrocarbon t a i l s by the f l u i d w i l l be s t r o n g l y d e n s i t y dependent; i t i s w e l l e s t a b l i s h e d t h a t t h e s o l v a t i o n o f simple nonpolar, h i g h e r molecular weight organic substances i n s u p e r c r i t i c a l f l u i d s i s h i g h l y density dependent (24). On a macro-scale, the degree of s o l v a t i o n of the m i c e l l e species w i l l a l s o be dependent upon pressure. By changing the pressure a t constant temperature, r e l a t i v e l y large changes i n the s o l v a t i n g power of the continuous phase occur, and the a t t r a c t i v e m i c e l l e - m i c e l l e i n t e r a c t i o n s can be strongly a f f e c t e d . We believe that as pressure i s lowered, so as t o approach the phase boundary, one f i n d s that these m i c e l l e m i c e l l e i n t e r a c t i o n s become i n c r e a s i n g l y important, u l t i m a t e l y l e a d i n g t o l a r g e - s c a l e a g g r e g a t i o n o f m i c e l l e s and phase separation (22). In t h i s paper we use dynamic l i g h t s c a t t e r i n g (DLS) methods to examine micelle s i z e and c l u s t e r i n g i n (1) s u p e r c r i t i c a l xenon, (2) n e a r - c r i t i c a l and s u p e r c r i t i c a l ethane, (3) n e a r - c r i t i c a l propane as well as (4) the l a r g e r l i q u i d alkanes. Reverse m i c e l l e or microemulsion phases formed i n a continuous phase of nonatomic molecules (xenon) are p a r t i c u l a r l y s i g n i f i c a n t from a fundamental viewpoint since both t h e o r e t i c a l and c e r t a i n spectroscopic studies of such systems should be more r e a d i l y t r a c t a b l e . Diffusion c o e f f i c i e n t s obtained by DLS f o r AOT microemulsions f o r alkanes from ethane up t o decane are presented and discussed. I t i s shown that m i c e l l e phases e x i s t i n e q u i l i b r i u m with an aqueous-rich l i q u i d phase, and that the apparent hydrodynamic s i z e , i n such systems i s highly pressure dependent.

Experimental The s u r f a c t a n t AOT ("purum" grade, Fluka) was p u r i f i e d as d e s c r i b e d by Kotlarchyk (2JL) . The AOT s o l u t i o n was f i l t e r e d through a 0.2-jxm M i l l i p o r e f i l t e r p r i o r t o drying i n vacuo f o r eight hours. The AOT was stored i n a d e s i c c a t o r over anhydrous calcium s u l f a t e . The molar water-to-AOT r a t i o (W) was assumed t o be 1 i n the p u r i f i e d , d r i e d s o l i d (25) . Water was d i s t i l l e d and f i l t e r e d through a M i l l i p o r e M i l l i - Q system. Ethane, propane ("CP" grade, Linde), and xenon (Research grade, Linde) were used as received. The alkanes had a reported p u r i t y of >99% (Aldrich) and were used as received. A c r o s s - s e c t i o n a l view of the high-pressure l i g h t s c a t t e r i n g c e l l i s shown i n Figure 1. The c y l i n d r i c a l l i g h t s c a t t e r i n g c e l l window was a high-precision sapphire tube with an i n s i d e diameter of 1.9 cm and an outside diameter o f 3.2 cm. To achieve the


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Figure 1. High pressure c e l l and holder f o r dynamic s c a t t e r i n g studies.

light


12.

Structure ofReverse Micdle and Microemulsion Phases

SMITH E T AL.

necessary alignment of the c e l l , the i n s i d e and outside surfaces of the sapphire tube are round and concentric t o within 0.0005 cm. The axis of t h i s tube was located within 0.002 cm of the axis of r o t a t i o n of the goniometer. The c e l l was placed i n an 9.25-cm diameter, thermostated quartz v a t f i l l e d with t o l u e n e . The p h y s i c a l dimensions of the s c a t t e r i n g sample c e l l were minimized to e l i m i n a t e convection o f the low-conductivity, l o w - v i s c o s i t y fluid. The t o t a l c e l l volume was 1.5 cm . The alignment of the instrument was confirmed using 58-nm polystyrene l a t e x spheres dispersed i n H 0, and the measured s i z e of t h i s standard was found to agree w i t h i n 5% of the reported value. In a d d i t i o n , our measurement of m i c e l l e s i z e f o r the H20/AOT/iso-octane system was within 5% of Zulauf and Eicke's (26) measurement f o r that system and showed the same functional dependence upon the W. Experiments with s u p e r c r i t i c a l xenon and ethane were done by adding 0.10 g of AOT (150 mM) into the s c a t t e r i n g c e l l with e i t h e r 16 μΐ water (W - 5) o r no added water (W - 1) . A miniature magnetic s t i r bar was also placed d i r e c t l y i n t o the s c a t t e r i n g c e l l . A Varian.8500 syringe pump f i l l e d with pure xenon or ethane was connected d i r e c t l y t o the s c a t t e r i n g c e l l . Upon p r e s s u r i z i n g with pure continuous phase, the s o l u t i o n was mixed f o r 15 min. and allowed t o e q u i l i b r a t e f o r one hour p r i o r t o data a c q u i s i t i o n . The o u t l e t of the s c a t t e r i n g c e l l was connected t o a pressure transducer (Setra Systems, No. 300C). Experiments with the other alkanes were conducted by f i l l i n g the s y r i n g e pump with the f i l t e r e d m i c e l l a r solution and then pumping t h i s s o l u t i o n d i r e c t l y i n t o the s c a t t e r i n g c e l l . The c o n c e n t r a t i o n of the alkane s o l u t i o n s was 0.015 mole of AOT/mole of alkane with a water-tosurfactant r a t i o of 5. A Malvern PCS-100 spectrometer (Malvern Instruments, Malvern, England) equipped with a 5 W A r l a s e r (488 nm) was used. The spectrometer and l a s e r were mounted on a v i b r a t i o n - f r e e o p t i c a l table (Technical Manufacturing Corp.). A l l measurements were taken at a constant s c a t t e r i n g angle of 54°. The s i g n a l from the p h o t o m u l t i p l i e r was processed on a 128 channel, r e a l time d i g i t a l c o r r e l a t o r (K7032-OS) using e i t h e r a 50 o r 100 nanosecond sample time. Temperatures were maintained with a Malvern temperature c o n t r o l l e r at 25 ± 0.1*C f o r experiments conducted i n xenon and the alkane l i q u i d s , and at 37 ± 0.1*C f o r experiments i n s u p e r c r i t i c a l ethane. The photon a u t o c o r r e l a t i o n f u n c t i o n was analyzed by the method of second order cumulants (22) i n which the logarithm of the normalized autocorrelation function, G (2.)(q,t), was f i t t e d t o a polynomial equation by using a nonlinear l e a s t squares f i t t i n g routine, 3


2

+

i

In [G (t)] - Γ - I \ t + j Γ ψ 2

0

(1)

2

where Γ i s i d e a l l y zero. Γ\, the f i r s t cumulant, d i f f u s i o n c o e f f i c i e n t , D , by σ

i s equal t o the

T

I \ - DT q 2

where q i s the s c a t t e r i n g vector.

(2)

The magnitude of the s c a t t e r i n g


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vector, q, i s given

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by

4π s i n ^6 Γ—*(3) λ/η where θ i s the s c a t t e r i n g angle, λ i s the wavelength of the incident beam i n vacuum, and η i s the index of r e f r a c t i o n of the s c a t t e r i n g medium. In the absence of i n t e r p a r t i c l e i n t e r a c t i o n s the second-order cumulant, Γ , i s r e l a t e d to the variance of the p a r t i c l e s i z e d i s t r i b u t i o n . The c o n t r i b u t i o n to the measured a u t o c o r r e l a t i o n function from the pure xenon or ethane f l u i d was determined to be n e g l i g i b l e because of the high signal-to-noise r a t i o (the scattered i n t e n s i t y at 54° of the xenon/AOT mixture was 10 times greater than for pure xenon). In addition, the measured a u t o c o r r e l a t i o n f u n c t i o n of pure xenon or ethane showed no a u t o c o r r e l a t i o n on the time s c a l e used to examine the m i c e l l e solutions. The mean (apparent) hydrodynamic diameter, d , was c a l c u l a t e d from D using the S t o k e s - E i n s t e i n relation for spherical particles q


2

H

T

d

— (4) 3π η D where k i s Boltzmann's constant, Τ i s the absolute temperature, and η i s the v i s c o s i t y of the solvent. The mean, ±1 standard d e v i a t i o n of f i v e r e p l i c a t e measurements, i s reported i n a l l cases. I t must be remembered that the reported hydrodynamic diameters may r e f l e c t d i f f e r e n c e s from the a c t u a l m i c e l l e diameters due to m i c e l l e - m i c e l l e i n t e r a c t i o n s . Large changes i n the v i s c o s i t y of near c r i t i c a l and s u p e r c r i t i c a l f l u i d s occur with moderate changes i n density. Since d i f f u s i o n c o e f f i c i e n t s w i l l be strongly e f f e c t e d by the f l u i d v i s c o s i t y i t i s e a s i e r to Interpret f l u i d s t r u c t u r a l changes by consideration of the data i n terms of the apparent hydrodynamic diameter. To c a l c u l a t e m i c e l l e s i z e and d i f f u s i o n c o e f f i c i e n t , the v i s c o s i t y and r e f r a c t i v e index of the continuous phase must be known (equations 2 to 4). It was assumed that the f l u i d v i s c o s i t y and r e f r a c t i v e index were equal to those of the pure f l u i d (xenon or alkane) at the same temperature and pressure. We believe t h i s approximation i s v a l i d s i n c e most of the d i s s o l v e d AOT is associated with the micelles, thus the monomeric AOT concentration i n the continuous phase i s very s m a l l . The density of s u p e r c r i t i c a l ethane at various pressures was obtained from i n t e r p o l a t e d values (23.) . Refractive indices were calculated from d e n s i t y values f o r ethane, propane and pentane using a semie m p i r i c a l Lorentz-Lorenz type r e l a t i o n s h i p (23.) . V i s c o s i t i e s of propane and ethane were c a l c u l a t e d from the f l u i d density v i a an e m p i r i c a l r e l a t i o n s h i p (30). S u p e r c r i t i c a l xenon d e n s i t i e s were i n t e r p o l a t e d from t a b u l a t e d values (31) . The Lorentz-Lorenz function (22.) was used to c a l c u l a t e the xenon r e f r a c t i v e i n d i c e s . V i s c o s i t i e s of s u p e r c r i t i c a l xenon (33), l i q u i d pentane, heptane, decane (2A), hexane and octane (15.) were obtained from previously determined values. H

T


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Results and Discussion Micelle-Micelle Interactions. Solvatochromic s t u d i e s have recently demonstrated f o r one-phase xenon and ethane systems that the aqueous core solvent environment of the reverse m i c e l l e s or microemulsion d r o p l e t s undergoes only very small changes f o r pressures from 200 bar (the approximate minimum pressure f o r formation of a one-phase system at [AOT]>10 mM) up t o 1500 bar (lu). Since we expect that the m i c e l l e s i z e and the core solvent environment t o be s t r o n g l y c o r r e l a t e d , we conclude that the p h y s i c a l s i z e of the s u r f a c t a n t / w a t e r assembly i s l a r g e l y independent of pressure. This i s supported by s p e c t r o s c o p i c studies of the iso-octane/AOT/water systems, i n which a d i r e c t c o r r e l a t i o n was established between m i c e l l e s i z e , as determined by neutron and l i g h t scattering (2 β 3 6 ) and t h e measured solvatochromic s h i f t (lu) . The e l e c t r o s t a t i c i n t e r a c t i o n s of the head groups and hydrogen bonding within the aqueous core c o n t r o l the s i z e of the nanometer-sized droplet, and not the properties of the nonpolar continuous phase solvent (except f o r r e l a t i v e l y small changes). This hypothesis i s f u r t h e r supported by studies i n l i q u i d alkanes and cyclohexane which showed l i t t l e or no e f f e c t of the continuous phase solvent on droplet s i z e (36). f

f

The bulk of the surfactant i n a reverse type microemulsion forms aggregate structures while a small amount of the surfactant, equal t o the c r i t i c a l m i c e l l e concentration (cmc), e x i s t s as d i s s o l v e d monomer or small c l u s t e r s i n the continuous phase. In l i q u i d systems, such as iso-octane (36), the cmc i s approximately equal t o 6 Χ 10" M. At s i m i l a r molecular d e n s i t i e s , iso-octane w i l l be a better solvent f o r surfactant monomer than e i t h e r ethane or propane because iso-octane has a much higher p o l a r i z a b i l i t y . In s u p e r c r i t i c a l f l u i d microemulsions the cmc ( i f such a d i s c r e t e value even e x i s t s ) has not been determined. However, since the cmc i s equal t o the s o l u b i l i t y of s u r f a c t a n t monomer i n the continuous phase, i t i s expected that the cmc w i l l be dependent on the f l u i d density ( i . e . , solvent strength) (JL2.) . Changes i n the cmc w i l l lead t o changes i n the s i z e of surfactant aggregates. In t h i s study, where the surfactant concentration i s greater than 10" M, these changes i n aggregate s i z e would be n e g l i g i b l e unless the cmc was unexpectedly much larger than 10~ M. 4

2

4

The general o b s e r v a t i o n from DLS s t u d i e s i s that the apparent hydrodynamic diameter increases as the pressure i s decreased towards a phase boundary (where surfactant and water w i l l p r e c i p i t a t e t o form a second phase) . Figures 2 and 3 show DLS r e s u l t s f o r AOT/water m i c e l l e s i n s u p e r c r i t i c a l xenon (at 25°C) and ethane (at 37°C), r e s p e c t i v e l y . Results are presented f o r [H 0]/[AOT] molar r a t i o s (W) of 1 (a) and 5 (b) . A l l measurements were obtained i n single-phase systems at constant W. The apparent hydrodynamic m i c e l l e diameter decreases with i n c r e a s i n g pressure o r density of the continuous phase i n both f l u i d s . The second cumulant i n Equation 1, which i s a q u a l i t a t i v e measure of the p o l y d i s p e r s i t y of the system, i s very close t o zero f o r a l l c o n d i t i o n s of t h i s study. There i s no s t a t i s t i c a l l y 2


17



3

Density (g/cm ) 2.2

100

200

2.4

300

400

500

600

700

Pressure (bar)

Figure 2. Apparent hydrodynamic diameters o f AOT reverse micelles i n s u p e r c r i t i c a l xenon as a function of pressure and density (of the pure f l u i d ) at 25°C, with (a) W - 1 and ( b ) W - 5. [AOT] - 150 mM.


SMITH E T A L .


3


Density (g/cm ) 0.36

0.40

200

0.44

300

0.48

400

500

600

700

Pressure (bar)

Figure 3. Apparent hydrodynamic diameters of AOT reverse m i c e l l e s i n s u p e r c r i t i c a l ethane as a f u n c t i o n o f pressure and density (of the pure f l u i d ) at 37°C, with (a) W « 1, and (b) W - 5. [AOT] - 150 mM.


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s i g n i f i c a n t e f f e c t of pressure on the second cumulant f o r the s i n g l e phase microemulsions formed i n xenon and ethane over the pressure ranges from 150 to 560 bar and W values from 1 to 5. The measured autocorrelation function of s u p e r c r i t i c a l xenon/AOT/water mixtures suggests that within the l i m i t s of d e t e c t a b i l i t y of the DLS technique, that a monodisperse m i c e l l e phase e x i s t s i n s u p e r c r i t i c a l xenon above 200 bar. The apparent hydrodynamic diameter of the AOT reverse m i c e l l e s i n s u p e r c r i t i c a l xenon i s l e s s than that found i n s u p e r c r i t i c a l ethane at equal pressures, and the e f f e c t of pressure on m i c e l l e s i z e i s l e s s than that i n ethane. This i s expected s i n c e the two-phase boundary f o r xenon i s at lower pressures than f o r ethane. The l i m i t e d data obtained from the s u p e r c r i t i c a l ethane m i c e l l e s o l u t i o n (37°C) at W » 5 i s due to the pressure c o n s t r a i n t s of our experimental apparatus and the r e l a t i v e l y high pressure at which t h i s s o l u t i o n ([AOT] - 150 mM) becomes one phase (450 bar). For both f l u i d s the s i z e appears to increase as the two-phase boundary i s approached. For W - 1 and W - 5 i n xenon, the asymptotic v a l u e s f o r the apparent hydrodynamic diameter at high pressure are approximately 3.0 and 4.3 nm; smaller than the s i z e of AOT reverse m i c e l l e s i n l i q u i d iso-octane (2) (3.8 and 5.6 nm) at the same water contents, but consistent with our DLS results f o r other l i q u i d alkanes presented later. Observations f o r the xenon system are complicated due to formation of the gas hydrate (clathrate) at higher pressures (15)β which may r e s u l t i n the a c t u a l m i c e l l e ( f l u i d phase) W value at higher water content being s l i g h t l y lower than expected. (The c l a t h r a t e s are observed as a s o l i d mass at the top of the high pressure c e l l , and we have found no evidence f o r corresponding structures d i s s o l v e d i n the f l u i d phase which might i n t e r f e r e with the present measurements.) Because the apparent m i c e l l e s i z e i n ethane changes sharply as the pressure i s increased, a s i m i l a r asymptotic value f o r m i c e l l e s i z e i s not reached even at the highest pressures examined. Solvatochromic probe studies of the aqueous core solvent environment show small changes i n m i c e l l e s i z e as a function of pressure which are due perhaps to the extent of solvent penetration i n t o the surfactant t a i l s (15.) · However, these changes represent only a small f r a c t i o n of the s i z e changes observed by DLS. The most reasonable explanation f o r the increase i n apparent hydrodynamic diameter measured by DLS i s the enhanced m i c e l l e m i c e l l e i n t e r a c t i o n s as the boundary of a two-phase system i s approached ( i . e . , the pressure i s lowered). Figure 4 i l l u s t r a t e s t h i s concept of m i c e l l e - m i c e l l e i n t e r a c t i o n s , which i s manifested as aggregation (or c l u s t e r i n g ) of the r e v e r s e m i c e l l e or microemulsion droplets. Since the s o l v e n t environment i s e s s e n t i a l l y unchanged by t h i s "macromolecular aggregation" (15.) we exclude the p o s s i b i l i t y of (other than transitory) m i c e l l e - m i c e l l e coalescence to form stable, l a r g e r m i c e l l e s . The m i c e l l e s may coalesce b r i e f l y to form t r a n s i t i o n a l species (which might be a "dumbbell" or more c y l i n d r i c a l s t r u c t u r e s ) , i n which the water cores c o l l i d e and intermix.


SMITH E T A L .

Structure ofReverse MiceUe and Microemulsion Phases


Reverse Micelle

AOT (bis(2-ethylhexyl)sulfosuccinate sodium salt)

MOLECULAR AGGREGATION !

Water core ι Polar head group Hydrocarbon tails

f }

Nonpolar fluid (continuous phase)

MRCROMOLECULRR AGGREGATION

mm /Xvxv/XvXjMIceller Aggregate ;X;

Figure 4. I d e a l i z e d AOT reverse m i c e l l e o r microemulsion s t r u c t u r e and a proposed a g g r e g a t i o n (or c l u s t e r i n g ) mechanism which maintains the d i s t i n c t solvent environments f o r the reverse m i c e l l e components.


175

176



Diffusion—Coefficients

of Reverse

Micelles

TECHNOLOGY

i n Liquid

and

S u p e r c r i t i c a l Alkanes. Figure 5 compares d i f f u s i o n c o e f f i c i e n t s f o r reverse m i c e l l e s (W - 5) i n alkanes ranging from propane to decane at 25°C and pressures up to 600 bar. The d i f f u s i o n c o e f f i c i e n t s measured i n l i q u i d s generally decrease by 10 to 15 percent as pressure i s increased to 400 bar, and show the expected systematic increase as alkane length decreases (and v i s c o s i t y increases). D i f f u s i o n c o e f f i c i e n t s of micelles i n near c r i t i c a l propane (25°C) and s u p e r c r i t i c a l ethane contrast with the larger alkanes by showing an i n i t i a l increase as pressure increased. This i s a s c r i b e d to the strong m i c e l l e - m i c e l l e i n t e r a c t i o n s i n these f l u i d s at lower pressures. The i n t e r a c t i o n s are r e l a t i v e l y small for propane (but s t i l l much greater than f o r the l a r g e r alkanes), r e s u l t i n g i n a maximum f o r the d i f f u s i o n c o e f f i c i e n t i n propane at -100 bar due to the opposing e f f e c t of increased v i s c o s i t y at higher pressures. The s u p e r c r i t i c a l ethane reverse micelles shown i n Figure 5, have d i f f u s i o n c o e f f i c i e n t s which i n c r e a s e with pressure and due t o the pressure limitations of our instrumentation the expected maximum was not observed. These r e s u l t s , again a t t r i b u t e d to micelle-micelle interactions, suggest that s i g n i f i c a n t l y improved mass transport properties f o r these systems are often obtained at higher pressures even though the v i s c o s i t y (of the f l u i d continuous phase) i s higher. F i g u r e 6 shows hydrodynamic diameters f o r the reverse m i c e l l e s i n l i q u i d alkanes at 25°C and s u p e r c r i t i c a l ethane at 37°C. The r e s u l t s show that m i c e l l e diameter i s generally i n the 4 to 5 nm range f o r the larger alkanes, although s l i g h t l y larger diameters were observed f o r decane. Generally, the larger alkanes show l i t t l e or no change i n hydrodynamic diameter with pressure, although the l a r g e r diameters f o r decane (and p o s s i b l y heptane) may suggest some micelle-micelle a t t r a c t i v e or more complex s t e r i c interactions. In contrast, propane and ethane show hydrodynamic diameters which decrease s u b s t a n t i a l l y as pressure i s increased, due to decreased micelle-micelle i n t e r a c t i o n s .

Hydrodynamic

Diameters

of Reverse

Micelles-Fluid—Ehaafia—in

E q u i l i b r i u m with Aqueous Phases. The formation and properties of reverse m i c e l l e and microemulsion phases i n e q u i l i b r i u m with a second predominantly water continuous phase i s of p r a c t i c a l i n t e r e s t f o r e x t r a c t i o n processes. Figure 7 compares apparent hydrodynamic diameters observed i n the ethane/AOT/water system at 37°C f o r values of 1, 3 and 16. In s i n g l e phase systems at W • 1 (a) and 3 (b) the apparent hydrodynamic diameter decreases with increased pressure due to decreased m i c e l l e - m i c e l l e i n t e r a c t i o n s as the solvent power increases. In contrast f o r a system with an o v e r a l l W - 16 (c) , where a second aqueous phase e x i s t s , hydrodynamic diameter increases continuously with pressure. Corresponding data f o r the propane/AOT/water system at 25°C are presented i n Figure 8 f o r W - 1, 5, and 20. In a single phase at W - 1 (a) hydrodynamic diameter i s n e a r l y i n v a r i e n t with pressure (3.8 ± 0.3 nm) with a s l i g h t increase suggested at the very lowest pressures. In a s i n g l e phase system at W • 5 (b),



12.

SMITH E T A L .


8.01 Supercritical Ethane

• /

Τ = 37°C Propane

n-Pentane iso-Pentane neo-Pentane n-Hexane n-Heptane iso-Octane n-Octane · — ^ — n-Decane

100

200

300

400

500

600

Pressure (bar)

Figure 5. Micelle diffusion coefficients f o r various alkanes as a function of pressure measured by DLS. Τ - 25°C, W - 5, YAOT • 0.015 (mole f r a c t i o n . )


SUPERCRITICAL FLUID SCIENCE AND T E C H N O L O G Y


12.0

10.01 Supercritical Ethane Τ = 37°C 8.01

φ φ

V

\

- Propane

ε .2 S

• A ο

6.0

υ I CQ c5K

"O Ο k. Ό

a

n-Pentane

A

iso-Pentane

Ο

n-Hexane

Δ

4.0

X

Δ

n-Heptane

•

iso-Octane

•

n-Octane

ο

n-Decane

2.0

0.0 100

200

300

Pressure

400

500

600

(bar)

Figure 6. Hydrodynamic diameters measured alkane/AOT/water s o l u t i o n s . Τ » 25°C, W - 5, (mole f r a c t i o n . )

by DLS Υ οτ Α


for 0.015

SMITH E T A L .



11.0

250

300

350

400

Pressure (bar)

Figure 7. Hydrodynamic diameters f o r s u p e r c r i t i c a l (37°C) ethane/AOT/water s o l u t i o n s as a function of pressure f o r W values of the o v e r a l l system o f l (a , 3 ( b - Q ) , and 16 (c - · ) .


17


180

SUPERCRITICAL FLUID SCIENCE A N D

TECHNOLOGY

greater m i c e l l e - m i c e l l e a t t r a c t i v e i n t e r a c t i o n s are evident f o r these m i c e l l e s with l a r g e r water cores leading to the expected a t t r a c t i v e increases at lower pressures. As shown previously i n Figure 6, the hydrodynamic diameter i s l a r g e r than the a c t u a l p h y s i c a l diameter (which we assume to be approximately equal to micelle s i z e i n the larger alkanes). The measured hydrodynamic diameters i n propane at W - 20 (Figure 8, c) show a maximum at -80 bar corresponding to the phase boundary f o r formation of a two-phase f l u i d - l i q u i d system. At lower pressures, the l i q u i d propane i s i n equilibrium with a lower predominantly water phase. Thus, as the phase boundary i s approached from higher pressures, m i c e l l e - m i c e l l e i n t e r a c t i o n s become i n c r e a s i n g l y important. As the phase boundary i s approached hydrodynamic diameters i n c r e a s e e x p o n e n t i a l l y and s u b s t a n t i a l l y i n c r e a s e d l i g h t s c a t t e r i n g i s observed at the detector. At the phase boundary the a t t r a c t i v e interactions cause a phase change where p o r t i o n s of both the AOT and water p r e c i p i t a t e t o form a predominantly water l i q u i d phase. Importantly, hydrodynamic diameters are s u b s t a n t i a l at pressures as low as 25 bar. While the actual m i c e l l e s i z e (W) has not yet been determined, i t i s apparent that the water to surfactant r a t i o w i l l vary continuously through the two-phase region. Further measurements of the m i c e l l e number density and p h y s i c a l diameters (W f o r the upper phase) are required to understand microemulsion formation and diameter, and hence solvent properties, i n the twophase region.

Conclusions The DLS r e s u l t s , taken i n conjunction with previous solvatochromic probe studies (15.), show the important r o l e of m i c e l l e - m i c e l l e i n t e r a c t i o n s i n determining both mass transport p r o p e r t i e s and phase behavior f o r reverse m i c e l l e and microemulsion systems. C l e a r l y m i c e l l e - m i c e l l e a t t r a c t i v e i n t e r a c t i o n s of the London-van der Waals type are of much greater importance i n n e a r - c r i t i c a l and s u p e r c r i t i c a l f l u i d s than i n l i q u i d solvents under the conditions studied. At low continuous phase d e n s i t i e s , s o l v e n t - m i c e l l e a t t r a c t i v e forces are lower and the lower d i e l e c t r i c continuous phase can l e s s e f f e c t i v e l y "shield'* m i c e l l e - m i c e l l e a t t r a c t i v e forces. These a t t r a c t i v e forces lead t o c l u s t e r i n g and, at low pressures, eventual coalescence of m i c e l l e s t o form e i t h e r a second reverse m i c e l l e phase or a second aqueous phase. In the two-phase region s i m i l a r c o n s i d e r a t i o n s apply, but here both m i c e l l e number density and [H 0]/[AOT] (W) i n the upper phase are v a r i a b l e with pressure. I t i s apparent that the a t t r a c t i v e i n t e r a c t i o n s , which cause the apparent hydrodynamic diameter to be l a r g e r than the p h y s i c a l diameter, are increased at both higher s u r f a c t a n t concentrations and l a r g e r W (at constant surfactant concentration). Experimental r e s u l t s , to be reported l a t e r (21), are consistent with t h i s expectation. These r e s u l t s suggest that f o r f l u i d s such as ethane and propane that the s i z e of the water core has a much greater e f f e c t upon m i c e l l e c l u s t e r i n g than the micelle concentration. Current r e s e a r c h i s aimed at a 2



12.

SMITH E T A L .


40.0

400

Figure 8. Hydrodynamic diameters f o r l i q u i d " n e a r - c r i t i c a l " (25°C) propane/AOT/water solutions as a function of pressure f o r W values of 1 (a - • ) , 5 (b - φ ) and 20 (c - Q ) .


181

182


quantitative understanding of the factors governing m i c e l l e number d e n s i t y and W i n the two-phase region and development of a p r e d i c t i v e model f o r phase behavior i n these systems.

Acknowledgment


Work supported by the Director, O f f i c e of Energy Research, O f f i c e of Basic Energy Sciences, Chemical Sciences D i v i s i o n of the U. S. Department of Energy (DOE) under contract DE-AC06-76RLO 1830. P a c i f i c Northwest Laboratory i s operated f o r the DOE by B a t t e l l e Memorial I n s t i t u t e .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Smith, R. D.; Fulton, J. L.; Gale, R. S. J. Am. Chem. Soc. 1987, 109, 920-921. Clarke, J.H.R.; Brown, D. J. Phys. Chem. 1988, 92, 28812888. Wong, M . ; Thomas, J. K . ; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730-4736. Fletcher, P . D . I . ; Parrott, D. J. Chem. Soc., Faraday Trans 1 1988, 84, 1131-1144. Hatton, T. A . ; Goklen, Κ. E. Separation Science and Technology, 1987, 22, 831-824. Dorsey, J. G.; Hernandez-Torres, Μ. Α.; Landy, J. S. Anal. Chem. 1986, 58, 744-747. Fendler, J. H. In Reverse Micelles, L u i s i , P. L., Straab, Β. E . , Eds.; Plenum: New York, 1984; 305-322. Evans, D. F . ; Sen, R.; Warr, G. G. J. Phys. Chem. 1988, 92, 774-783. Eicke, H. F . ; H i l f i k e r , R.; Kim, V. J. Colloid Interface S c i . 1988, 121, 579-584. Eicke, H. F . ; H i l f i k e r , R. J. Chem. Son., Faraday Trans. 1 1987, 83, 1621-1629. Huang, J. S. J. Chem. Phys. 1985, 82, 480-484. Roux, D.; Bellocq, A. M. In Surfactants in S o l u t i o n , M i t t a l , K. L., Lindman, B . , Eds.; Plenum: New York, 1984; 1247-1261. Smith, R. D.; Fulton, J. L. J. Phys. Chem. 1988, 92, 29032907. B l i t z , J. P . ; Fulton, J. L . ; Smith, R. D. J. Phys. Chem. 1988, 92, 2707-2710. Fulton, J. L . ; Blitz, J. P. Tingey, J. M. Smith, R. D. J. Phys. Chem., in press. Smith, R. D.; Fulton, J. L. In: Surfactant-Based Mobility C o n t r o l , ACS Symposium Series, 373, Smith, D. H . , Ed.; American Chemical Society: Washington, D. C., 1988. Prausnitz, J. M . ; Randolph, T. W.; Clark, D. S.; Blanch, H. W. Science 1988, 238, 387-390. Beckman, E. J.; Smith, R. D. J. Phys. Chem. in press. Matson, D. W.; Fulton, J. L . ; Smith, R. D. Mat. Lett. 1987, 6, 31-33. Gale, R. W.; Fulton, J. L . ; Smith, R. D. Anal. Chem. 1987, 59, 1977-1979.


12. SMITH ET AL. Structure of Reverse Micdle and Microemulsion Phases 183

21. 22. 23. 24.


25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Smith, R. D . ; Gale, R. W.; Fulton, J. L. LC•GC 1987, 6, 134-142. Smith, R. D . ; Fulton, J. L . ; Blitz, J. P.; Tingey, J. M. J. Phys. Chem., submitted. Gruen, D.W.R.; Haydon, D. A. Pure &Appl. Chem. 1980, 52, 1229-1240. Schmitt, W. J.; Reid, R. C . J. Chem. Eng. Data 1986, 31, 204-212. Kotlarchyk, M.; Chen. S.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054-2069. Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480-486. Koppel, D. E. J. Chem. Phys. 1972, 57, 4814-4815. Younglove, B. A.; Ely, J. F. J. Phys. Chem. Ref. Data 1987, 16, 577. Hardich, J. J. Chem. Phys. 1976, 64, 2265-2266. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. In The Properties of Gases and Liquids, 3rd ed.; McGraw Hill: New York, 1977, p. 426. Michels, Α.; Wassenaar, T.; Louwerse, P. Physica XX 1954, 99-106. Smith, B. L . ; Parpia, D. Y. J. Phys. C: Solid St. Phys. 1971, 4, 2251-2257. Thodos, G.; Shimotake, Η. Α.I.Ch.E Jour. 1958, 4, 257-262. Stephen, Κ.; Lucas, Κ. in Viscosity of Pure Fluids, Plenum Press: New York, 1979. Brazier, D. W.; Freeman, G. R. Can. J. Chem. 1969, 47, 893899. Kotlarchyk, M.; Huang, J. S.; Chen, S. H. J. Phys. Chem. 1985, 89, 4382-4386. Fulton, J. L . ; Smith, R. D. J. Phys. Chem., to be submitted.

RECEIVED May 1, 1989


Structure of Reverse Micelle and Microemulsion Phases in Near

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